Police believe the publishers of the News of the World were involved in hacking phones as recently as 2009, the Leveson Inquiry into press standards heard today.

Private detective Glenn Mulcaire's notebooks suggest that at least 28 News International employees commissioned him to illegally intercept voicemails, the hearing was told.

Records of his hacking activities also include references to "the Sun" and "Mirror", the inquiry heard.

Mulcaire was jailed along with the News of the World's former royal editor Clive Goodman in January 2007 after they admitted intercepting voicemail messages left on phones belonging to royal aides.

Detectives seized 11,000 pages of the disgraced private investigator's notes, in which he often wrote a first name or "private" in the top left-hand corner.

The 28 names which are legible in the papers correspond to News International employees, one of whom apparently made 1,453 separate requests for information from the private investigator.

Prime Minister David Cameron set up the Leveson Inquiry in July after revelations that the News of the World hacked the phone of murdered schoolgirl Milly Dowler after she went missing in 2002.

The inquiry, which formally began hearings at the Royal Courts of Justice in central London today, was told Scotland Yard detectives have uncovered evidence that the practice was still going on seven years later.

Robert Jay QC, counsel to the inquiry, said: "According to the Met Police, News International's hacking operation had certainly begun by 2002, Milly Dowler being the first known victim. The police believe that it continued until at least 2009."

The inquiry heard suggestions that another journalist may have been working with Goodman to hack royal phones.

Mulcaire's notes relating to the voicemails of one royal aide were marked with the name of a News International employee who was identified only as "A" for fear of prejudicing the ongoing police investigation into the scandal.

Mr Jay noted: "One possible inference to be drawn is that 'A' was working with or for Goodman, and he or she may have instructed Mulcaire to carry out an interception."

Mr Jay said the private detective's guilty plea to hacking the phones of people who were not royal aides should have alerted News International to the fact that the practice went beyond Goodman.

"The five individuals I mentioned in the context of these counts would not have been of interest to the royal editor," he said.

"This must have been obvious to News International at all material times, by which I mean anyone within the company equipped with a basic familiarity with these facts."

Mulcaire's notebooks relate to a total of 2,266 taskings and include the names of 5,795 potential victims, the inquiry heard.

"A" requested information from the private investigator on 1,453 occasions, followed by other News International employees identified as "B" (303 times), "C" (252) and "D" (135).

Mulcaire's notes for Mr Clifford and Mr Taylor refer to "A", his notes for Mr Hughes refer to "A", "B" and "C", and his notes for Ms Macpherson refer to "B", the inquiry heard.

News International's records show that "Alexander" - a pseudonym for Mulcaire - was paid between November 2005 and August 2006 for information relating to "Fergy", "SAS", "Wills" and "Harry and Chelsy".

Mr Jay said: "The scale of this activity gives rise to the powerful inference that it must have occupied Mulcaire full time."

The barrister rejected the defence previously mounted by News International that hacking at the News of the World was limited to a single "rogue reporter".

"It's clear that Goodman wasn't a rogue reporter. Ignoring the 'private' corner names and the illegible, we have at least 27 other News International employees," he said.

He added: "I suggest that it would not be unfair to comment that it was at the very least a thriving cottage industry."

Mr Jay said it appeared that the illegal interception of voicemails went beyond the News of the World.

"The inquiry is beginning to receive evidence to indicate that phone hacking was not limited to that organisation," he said.

The hearing was told that actor Jude Law has brought a claim against The Sun for allegedly hacking his phone.

Mr Jay said: "Part of the evidential matrix in support of his case is a corner name in the Mulcaire notebook which simply states 'the Sun' without specifying the individual working there."

He added: "There is also documentary evidence which we have seen of another corner name relating to the Mirror."

Lord Justice Leveson stressed that the freedom of the press was "fundamental" to the UK's democracy and way of life but said it must be exercised "with the rights of others in mind".

He said the task of his inquiry could be summed up in one simple question: "Who guards the guardians?"

The first part of the inquiry is looking at the culture, practices and ethics of the press in general.

The second part, examining the extent of unlawful activities by journalists, will not begin until detectives have completed their work and any prosecutions have concluded.

Responding to the suggestion that Mulcaire may have hacked phones for the Mirror, a Trinity Mirror spokesman said: "It was not made clear to the inquiry which newspaper he was referring to nor did he elaborate on the nature of the 'evidence' or how it 'relates' to the Mirror.